Those books were meant to increase the reader's understanding of why different positions are better or equal, thus not just an evaluation but the problem-solving methodology. In other words, the positional understanding as well as the technique involved in the solutions.
You don't find "modern" openings there because, at that time, those systems were regarded as "suspicious" or "inferiors" because of different reasons. Thing is, most of what we do in chess is related to what we "believe" will work in the long term because we cannot calculate all the consequences. Some people don't go to school and become millionaires, and some have several degrees and can barely pay rent... however, in our societies this is often the way around, the reason why most of us go to school. And that is the equivalent of chess positional understanding.
It's not just one book , but several classical books with games annotated.
For example books like :
Zurich 1953 Chess Tournament book
alexander alekhine international tournament in new york 1924-1927 book
three hundred chess games tarrasch book
Akiba rubinstein book
Karpov books
Take even Kasparov's book my great predecessors
HOW do I study them if I play SCANDINAVIAN DEFENCE for black against E4 and Sokolsky opening for white !? I mean sure I still can benefit from those games looking at different openings and learning structures but that's a lot of information trying to learn every single opening and just waist of time really I won't remember them all I need to concentrate on my openings more !
For example I play SLAV or semi slav defence agaisnt d4 so ALL THOSE books are very helpful for me because I can see hundreds or even more games just on slav or semi slav and learn even better ideas structures etc... So what about the rest if I won't play them all !?
My second question would be why none of the books have opening like scandinavian defence or birds opening or sokolsky etc.... ?